Method and apparatus for recovering methane from hydrate near the sea floor

ABSTRACT

Method and apparatus are disclosed for recovering natural gas from a hydrate deposit near or at the sea floor. A rigid dome structure, supported by a floating vessel and movable along the sea floor, is placed over the hydrate deposit. Heated sea water may be pumped from the floating vessel through headers and nozzles in the rigid dome structure. The heated sea water preferably fluidizes the marine sand containing hydrate, releasing gas in the hydrate. The gas is collected in the rigid dome structure and moved through a pipe to the floating vessel, where it may be processed through a turbo-expander and liquefied.

This regular U.S. application claims priority to U.S. provisional application Ser. No. 61/437,099, flied on Jan. 28, 2011.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to recovery of methane gas from hydrate deposits on and very near the seafloor. More particularly, a system including a dome structure for capturing the gas and a system of hot-water nozzles for fluidizing the sediments and releasing the gas is disclosed along with processing steps and apparatus to recover energy from and liquefy the released gas.

2. Description of Related Art

The dissociation of methane gas hydrate is represented by:

(CH4·6H2O)solid→(CH4)gas+6(H2O)liquid

The energy value of the produced gas is ten times what is needed for the dissociation of the hydrate.

A recent article (R. Boswell, Science, August, 2009) reports that the global resource of methane in gas hydrate deposits is commonly cited as 20,000 trillion m³. For reference, the annual natural gas use in the United States is currently just over 600 billion m³. Much of the gas in hydrates is in rock formations remote from the surface of the earth or the sea floor. The methods disclosed herein apply only to the methane in a porous medium on or near the sea floor. The porous medium is referred to herein as “silt” (or “marine sand”). Marine sands are a significant source of gas hydrates in the Gulf of Mexico, according to the U.S. Minerals Management Service.

Oil and gas continually migrate from great depths in the earth's crust toward the surface in some areas of the earth. When migration is under an ocean, some of the oil and gas is trapped remote from the seafloor, but much migrates to the seafloor, In deep water, hydrates form at the seafloor. For this reason, natural oil and gas seeps and their associated hydrates are studied as a guide to the presence of subsurface fields. Methane hydrate often occurs near existing offshore platforms for oil or gas production. For example, gas hydrates near the sea floor were discovered near the Jolliet Field in the Gulf of Mexico, near a site now occupied by a huge oil platform.

If hydrates decompose or disassociate, methane is released into the sea and then into the atmosphere. Disassociating and capturing gas hydrates from the sea floor may prevent release of methane into the atmosphere. Using hydrate methane industrially as a fuel would convert it to carbon dioxide, actually decreasing the short-term effect on atmospheric chemistry and global change. In addition, methane may be used as a chemical feedstock and is an environmentally cleaner fuel than oil, coal, or oil shale, which all have an environmental impact during production and combustion.

Hydrocarbons captured in gas hydrates come from various sources deep in the earth. In addition, bacteria in marine sediments naturally produce enormous volumes of methane when they feed on plant debris washed into the gulf from rivers and swamps. This “biogenic” methane is often trapped in layers of hydrate that simulate the contours of the seafloor, and can be detected by various geophysical techniques commonly used in oil and gas exploration. Monitoring devices can be utilized to verify production and prevent plugging from gas hydrate formations, as well.

The article “Preliminary Evaluation of In-Place Gas Hydrate Resources: Gulf of Mexico Outer Continental Shelf” documents the U.S. Minerals Management Service's (MMS) efforts in developing a model for performing assessments of potential quantities of methane hydrates located on the Outer Continental Shelf (OCS). This initial US-funded research effort focused on modeling in-place quantities of biogenically-sourced gas hydrate in the Gulf of Mexico (GOM). The report discloses that gas hydrates can form in water depths in excess of about 400 meters, provided that an adequate supply of methane is available and that temperature and salinity are not excessive. However, the thickness of the hydrate stability zone in the GOM was determined to not significantly exceed 1000 meters because of temperatures too high for formation of gas hydrates. The MMS model predicted that 6,717 trillion cubic feet (TCF) of methane gas resides in sandstone reservoirs in the GOM, and 14,727 TCF resides in shale and fractured reservoirs in the GOM. (One cubic foot of gas hydrate at reservoir temperature and pressure yields approximately 160 cubic feet (ft) of methane gas at atmospheric standard conditions.)

Elliot et al. (U.S. Pat. No. 4,376,462) discloses pumping relatively warm brine water down to hydrates under a body of land or water through a conduit, allowing the brine to circulate through the hydrates to melt and produce gaseous hydrocarbons, and then separating gaseous hydrocarbons from the spent brine. The patent discloses how to harvest gas hydrates from solid concentrations of gas hydrates, but does not disclose how to harvest gas hydrates from the pore spaces of marine sands on the sea floor by fluidizing the marine sands.

Agee et al. (U.S. Pat. No. 5,950,732) discloses a means to harvest gas hydrates by use of heat or depressurization immediately above the surface of the gas hydrates. This patent does not disclose a means to fluidize the marine sands and gas hydrates in the pore spaces of the marine sands on the seafloor to allow rapid dissociation and efficient, safe collection and transportation to the surface for expansion, cooling and storage

Nohmura (U.S. Pat. No. 6,192,691) discloses a flexible sheet that is sunk to the sea floor to trap methane hydrate gas and that fills by the buoyancy of the gasified methane. However, this patent does not provide a means to fluidize the marine sands and gas hydrates found in their pore spaces

Wyatt (U.S. Pat. No. 6,299,256) discloses a flexible cover with steerable pods with a mining module connected to an inside surface of the cover to dislodge deposits by mechanically agitating and/or heating and thawing. However, this patent relies on a flexible cover to capture the released methane gas without any structural members to avoid collapse of the flexible cover from variations of up and down currents on the seafloor (either naturally or artificially occurring). Additionally, this patent relies on mechanical equipment being operated up to a mile below the sea surface to move the cover and mine the gas hydrates. A method to fluidize the marine sands or to control the flow of methane gas to prevent two-phase flow in the pipe to the surface is also not provided.

Marine sands are a significant source of gas hydrates in the GOM, as estimated by MMS, and are considered more economically feasible to harvest. What is needed is apparatus and method for economically recovering the methane gas bound in a hydrate in sediments at and near the seafloor.

BRIEF SUMMARY OF THE INVENTION

Method and apparatus for collecting methane in a moveable dome structure are disclosed. The methane is released from hydrate in marine sands or solid formations by warm or hot water or steam jets impinging on the sea floor. The methane may be piped upward and expanded through a turbine on a vessel to produce electrical power. The methane may then the liquefied and stored on the same or another vessel.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic sketch of one embodiment of the process disclosed herein.

FIG. 2 is a drawing of apparatus used at the seafloor and at the sea surface.

FIG. 3 is an isometric view of a connected array of modular pyramid-shaped dome structures near the seafloor with piping for contacting the sediments with warm water from nozzles and collecting gas released from hydrate near the seafloor. Note that it might be preferable to place the piping in a location other than the center of the dome, e.g. at the leading edge of the dome.

FIG. 4 is a diagram of the recovery process for which thermodynamic calculations may be performed to predict and optimize conditions for gas recovery.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an overview of apparatus 10 used in one embodiment of the process disclosed herein. One or more dome structures 11 are lowered to a depth near the seafloor where hydrate exists. Heater 12 on ship 20 heats seawater and pump 13 pumps the hot seawater through control valve 14 and water pipe 15 to spray header 16. The spray header distributes warm water through spray jet nozzles, preferably evenly spaced and optimized to fluidize the marine silt and sand to release solid gas hydrates located near or on the seafloor. Gas from hydrate collects under dome 11 and migrates to the top to form gas space 17. Gas flows to the surface through gas pipe 19. Valve 19A controls the rate of upward flow of methane gas to control the size of the gas space and to avoid two-phase liquid and gas flow in pipe 19. Cables C1 are used to hoist, lower and move dome 11. Although a ship is illustrated in the figure, any structure at the surface may be used, such as a semi-submersible platform.

Gas hydrates are fluidized and heated by the warm or hot water coming from sprays located at the bottom of the dome structure. The warm water sprays efficiently release the gas hydrates from within the pore spaces of the marine sands into a fluidized state with the surrounding hot water. When harvesting from solid gas hydrate formations, the warm water sprays may gently release the top surface of gas hydrates with minimal impact on surrounding marine life. Flow from the warm water sprays also provides a lift force so that the dome is lifted off the sea floor and can slowly travel forward to harvest gas hydrates on a continuous basis. Horizontal jets 18 may be used to propel dome 11 along the seafloor. Jets 18 may be placed on each side of dome 11 and flow rate through the jets may be independently controlled to move the dome in a preferred direction. Video cameras 11A may be used to identify obstacles ahead for adjustment in course of dome 11 and for monitoring operations in and around dome 11. Minimal forward movement may be required when harvesting from solid gas hydrate formations. It should be understood that other methods may be used for propulsion of the dome structure. For example, it may be moved by cables from a ship or platform that drags it along the bottom on sled runners. Propulsion by propellers or other methods may be used instead of or along with jets.

Methane gas liberated from the dissociation of the gas hydrates may travel to the surface through gas pipe 19. Gas pressure at the surface will be near the pressure at the seafloor where it is liberated, which is the hydrostatic head of seawater at that depth. The high pressure of the methane gas (almost 0.5 psi per foot of water depth) is then let down to near atmospheric pressure by expansion of the gas through turbo-expander T1. Expander T1 performs work, preferably by generating electrical power with generator E1. The letdown of the pressure through the expander connected to an electrical generator provides cooling to the methane gas necessary for liquefying some of the gas. Additional refrigeration may be supplied by use of some of the electrical power. Liquefaction technology is well known in the natural gas industry. The Linde liquefaction process is described, for example, in J. M. Smith et al, Intro. to Chem. Engr. Thermo., 7^(th) Ed., 2005, pp. 326-329. Storage in the liquid state is provided in tanks S1. Electrical power may be used in other topside operations and in supplemental heating in the gas hydrate harvesting dome.

FIG. 2 illustrates a preferred embodiment of apparatus for harvesting gas hydrates on the seafloor. Ship 20 is equipped with T1, E1 and S1, as described above. Water line 15 and gas line 19 have been used, along with support cables (not shown) to place dome array 22 on the seafloor where gas hydrate is present. Twenty-live domes are illustrated in array 22, but the array may contain any desired number of domes. The top section of the dome structures preferably includes insulation to prevent re-formation of gas hydrates. The water pipe 15 and gas pipe 19 are shown separately and also require insulation to reduce heat loss. Alternatively, the pipes may be concentric, with water pipe 15 being on the outside to lower heat loss from the methane gas to prevent reforming of gas hydrates from any residual moisture carryover into the up-flow stream.

FIG. 3 is an isometric view of a module containing twenty-five domes 11 and showing water headers 16 and gas headers 30. Gas pipe 19 and water pipe 15 connect the headers to ship 20 (not shown). Also shown are nozzles 34 connected to water header 16. Skirt 32 is attached to the lower edge of array 22. Flexible connecting material 36 is preferably used to join domes into array 22. Cameras 11A preferably allow observation in each direction around the module. One camera would preferably be directed in the direction of movement of the array.

The design of the gas hydrate harvesting dome structure illustrated in FIGS. 2 and 3 reduces the amount of water that would need to be displaced on start-up of the operation. Additionally, the modular design of a gas hydrate harvesting dome allows for flexibility in the structure to conform to variations in the seafloor topography. Outer skirt 32, preferably made of a flexible seal material, will also conform to the sea floor topography and will separate the warm water under the combined dome structure from the cold, exterior seawater.

Movement of the gas hydrate harvesting dome on the seafloor is facilitated by the lift of the warm water spray nozzles and methane gas so that the gas hydrate dome is preferably suspended above the seafloor by about. 1-2 feet. Warm water spray nozzles 18 (FIG. 1) would be positioned in horizontal positions on all sides to provide the motive force for movement in any direction to achieve the desired production rates. Forward speed would be controlled by the flow of the warm water sprays in the rear while braking would be accomplished by the flow of the warm water sprays in the front of the dome. Flow into warm water sprays on either side would be used for steering and changing direction.

Warm or hot water is pumped to the water spray headers located at the bottom of the harvesting dome structure, as explained further below. A flexible grid design of the warm water spray header provides the ability of the spray header to conform closely to changes in terrain and elevation of the seafloor to insure close contact and efficient fluidization of the marine sands and gas hydrates, preferably to a depth of 6 inches or greater into the seafloor. When harvesting from solid gas hydrate formations, the harvesting dome will conform closely to the surface of the formation and may remain stationary in one location for a time. It may then be moved to an adjacent location, or it may be moved continuously. The rapid fluidization of the marine sands by water spray jets allows for highly efficient heat transfer, which is required for the endothermic dissociation reaction to produce methane gas. This provides significantly greater heat transfer versus the dependence on conductive heat transfer alone through the marine sands, which have very low thermal conductivity—similar to most insulation materials.

The design of the gas hydrate harvesting dome structure is important for the production rate for the harvesting of gas hydrates. For a topside vessel of 1000 ft. length, a gas hydrate dome size of 500 ft wide and 500 ft long has been used as the design basis. An efficient spray header design needs to take into consideration the forward movement of the gas hydrate dome on the seafloor to efficiently provide surface coverage. The preferred approach for harvesting from marine sands is to utilize fan spray nozzles positioned perpendicular to the forward movement of the dome, which provides both impact energy and wide coverage for efficient fluidization and heat transfer. For harvesting from solid gas hydrate formations, a wide cone spray design is preferred to provide even heating while lowering the impact energy on the surface of the solid gas hydrates with minimal negative impact on surrounding marine life. A wide cone spray pattern spreads out the total flow through the spray over a wider impact area to minimize the impact force produced per square foot of the surface beneath the spray nozzle

Another important design consideration is the time that each location of marine sands is exposed to the warm water under the dome structure to fluidize and dissociate the gas hydrates in marine sands. Considering a forward movement of 0.5 ft/see, a dome structure 500 ft. long in the direction of movement would provide 1000 seconds residence time for fluidization, heating and dissociation of gas hydrates into methane gas (16.7 minutes). With the use of 5 spray header arms starting at the forward, leading edge and evenly spaced across the length of the dome, the marine sands would be forcefully fluidized and reheated every 200 seconds (3.3 minutes). With a lift force in the dome designed to lift the dome 12 inches from the pressure of the warm water spray headers, a nozzle spacing of 18 inches would provide 100% coverage on the seafloor and require 333 spray nozzles per arm and 1665 spray nozzles total.

With a design flow of 7.1 gal/min per nozzle, resulting in 500 psi net positive force applied at the discharge of the spray nozzles, a total flow 11,821 gal/min would be required. Sizing and design of the warm water spray header using fluid flow simulation software indicates that two 12-inch diameter supply pipes would be required for a warm water spray header of this size, reducing down to an 8-inch diameter main pipe header size and 6-inch diameter headers perpendicular to the forward movement of the dome. The weight of the gas hydrate dome to overcome buoyant forces for the chosen depths of gas hydrate harvesting needs to be factored into the design flow and pressure output of the warm water spray header nozzles. The Objective of the warm water spray header design when harvesting from marine sands is to achieve the desired fluidization of the gas hydrates to achieve targeted dissociation rates and sufficiently lift the gas hydrate dome to achieve forward movement with minimal applied force. The mechanical design of the dome structure could be of stainless steel plate, which may have an attached insulating layer, and frame construction or other suitable materials to provide the necessary strength and weight required.

To reduce start-up heating time and maximize hot water temperatures that can be achieved beneath the gas hydrate dome, a low-profile design of the harvesting dome would be utilized. For a 50 ft high pyramid design and 500 ft wide by 500 ft long, the total volume beneath the dome would be 4.17×10⁶ cubic feet (31.17×10⁶ gallons). Assuming an incoming temperature of 300 deg F. of water and a rate of 11,821 gal/min, it would take approximately 264 minutes (4.4 hours) to replace 10% of the water under the dome with the higher temperature water to achieve a suitable warm water temperature. If the water under the dome is at 35 deg F. and assuming ideal mixing with the assistance of an efficient warm water spray water header design, the temperature under the dome would reach approximately 61.5 deg F. 4.4 hours [0.9×(35+460)+0.1×(300+460)=521.5 deg R=61.5 deg F]. The temperature of the warm water under the harvesting dome would increase with time and with efficient retention of warm water by the use of side-skirts 32 (FIG. 3) on the harvesting dome structure. This will form a seal between the seafloor and the harvesting dome to prevent outside cold water from penetrating inside the dome from forward movement of the dome and increase production rates from the dissociation of gas hydrates into methane gas.

The gas hydrate harvesting dome may also be positioned over methane gas seeps coming from the seafloor or any other methane gas collected without any dissociation of gas hydrates required. The warm water sprays would help prevent the formation of gas hydrates, as well. Furthermore, the invention as disclosed or claimed can be used to help recover crude oil on the seafloor by heating the crude oil with the warm water sprays to cause the oil to rise to the surface (which may require supplemental pumping, as well). It may also be placed on a hydrate “hill” in stationary mode to release the methane.

It is seen that the thermal diffusivity of sand is very low (around 6×10⁻⁷ as); thus, heat transfer through conduction is very inefficient. If the water temperature at the seafloor surface is 100 deg F., in 40 minutes the calculated temperature 6 inches below the seafloor surface would rise only by approximately 0.3 deg F. Other calculations were also made with assumed seafloor surface temperatures of 150 deg F. and 200 deg F. No significant increase of heat transfer rate was predicted. Therefore, it is concluded that heating of marine sands to dissociate gas hydrates is not practically possible without fluidization to provide direct contact of the hot water with the gas hydrates, thereby removing the insulation effect of the marine sands.

Storage of methane gas may be provided on a separate vessel designed specifically for this purpose. This would allow processing of the methane gas from high-pressure gas to low pressure, forming refrigerated liquid on the primary vessel while storing the liquid on transient vessels. This would require a transfer line to the secondary, transportation vessel. Land-based, robotic, remote control of the processing and storage facilities disclosed herein may also be provided.

For a square dome with a fluidization depth of 6 inches, the total volume of marine sands and gas hydrates that would be fluidized would be 125,000 cubic feet (500 ft.×500 ft.×0.5 ft.). Assuming 30% by volume of the marine sands contain gas hydrates, the total volume of gas hydrates would be 37,000 cubic feet (125,000×0.3). For complete dissociation of these gas hydrates into methane gas, a volume of 6 million cubic feet of methane gas at standard atmospheric conditions would be produced. With the speed of the dome traveling at 0.5 ft/see, it would take approx. 16.7 minutes to produce 6 million cubic feet of methane gas which translates into a production rate of approx. 360,000 standard cubic feet per minute (SCFM) of methane gas. The size of the pipe recommended for the venting of the methane gas produced from the dissociation of gas hydrates would be 12 inch diameter with a total pressure drop of approx. 38 psig for a 5000 ft. riser to the topside operations as shown in Exhibit B.

Based on the depth of the gas harvesting operation on the seafloor, the pressure of the methane gas delivered to the topside operations would vary from 1500-2000 psig. With the use of a process simulator, it can be determined that an expander connected to an electrical generator would allow for efficient let-down of the methane gas pressure to near atmospheric pressure and cooling to approximately −259° F. and provide for cost efficient storage in the liquid phase, as shown in FIG. 7.

Additional processing for removal of water and contaminants is not anticipated due to the purification and concentration of methane gas in hydrates with increasing depth of their formations. The methane gas produced for this invention as claimed would be commercial quality ready for use in the natural gas distribution grid.

With an energy value of 1000 Btu per cubic feet of methane gas and market price of $4 per million Btu, the total value of this production rate would be $724,000,000 per year for a 350 day, 24 hour per day operation as shown below:

6,000,000 ft3/min×1000 Btu/ft3×$4/million Btu/16.7 min×60 min/hr.×24 hr./day×350 day/yr. $724,000,000/yr.

The value of the energy produced from the letdown of the methane gas pressure from the expander would be as follows:

106,500 kilowatts×24 hr./day×350 days/yr.×$0.15/kilowatt-hr.=$134,190/yr.

The economic benefits of this invention as claimed are quite significant from the above sample design case. The production rates of methane gas produced are directly influenced by the efficiency and depth of penetration of the warm water spray nozzles to fluidize and heat the gas hydrates trapped in the pore spaces of the marine sands or to melt solid formations of gas hydrates. Optimization of the penetration depth, fluidization and heat transfer of the hot water and the gas hydrates will further increase the production rates and economic benefits.

The most exciting aspect of this invention as claimed is that the methane gas collected can be directly used as a clean energy source without any further refining or processing required as is now required with crude oil collected from offshore rigs. Additionally, there are significantly less negative environmental impacts on the water and air quality due to the less intrusive, surface harvesting approach of this invention as claimed versus reliance upon drilling into pressurized reservoirs beneath the seafloor.

Storage of the methane gas on the topside vessel would be at near atmospheric pressure in liquid state as is the common design practice for storage of methane gas on seagoing vessels. The topside operations would be located immediately above the gas hydrate harvesting dome. Additional storage vessels could be docked nearby for receiving and transporting methane gas to the mainland.

The size of the gas hydrate harvesting operation can be from small- to large-scale, depending upon the desired investment and return desired. Multiple topside vessels can be used in adjacent paths to increase the overall production rates.

As an alternative of the same invention, the methane gas produced from the harvesting dome could be routed via undersea pipelines to a land-based operation for depressurizing and liquefying the methane gas.

FIG. 4 illustrates thermodynamic calculations that are used to predict performance and optimize conditions in the process disclosed herein for gas hydrate harvesting. Methane gas dissociated at the sea floor (condition A) flows through a 12 in pipe to the topside vessel (condition B), where it is passed through a turbine (under condition C) to extract energy by simple expansion to atmospheric pressure. In this process the gas cools down to −258.676 deg F., which can be further liquefied by just cooling it to −259 deg F. (condition D). Conditions used to calculate the amount of energy generated by the turbine are shown in Table 1, which is an example flow sheet run assuming:

Flow rate: 100 Ibmol/hr. (0.9 MMSCFD)

Composition: 100% Methane

The gas at the inlet of the turbine is at 2033 psia and 58 deg F. When it expanded at atmospheric pressure by passing through a turbine, 0.6 MMBTU/hr. of energy is generated. Table 1 shows conditions from the seafloor to liquid methane. Current design suggests that around 500 MMSCFD of methane gas can be generated every day (24 hrs.), which results in 363 MMBTU/hr. (106.5 MW).

TABLE 1 Gas through Name pipe AT SEAFLOOR Temperature F. 70 Pressure Psia 2415 Mole Flow, lb-mol/hr 100 Volume Flow, ACFM 3.28291 Mass Flow, lb./hr. 1604.28 GAS AT SHIP Temperature F. 58.8674 Pressure Psia 2033 Mole Flow, lb-mol/hr. 100 Volume Flow, ACFM 3.74038 Mass Flow, lb./hr. 1604.28 THROUGH TURBINE Polytropic Efficiency 0.68495 Gas Power, BTU/hr. −654649 Polytropic Head, ft-lbf/lbm −110307 Temperature, F. −258676 Pressure, psia 14.7 GAS OUT OF TURBINE Temperature F. −258.676 Pressure Psia 14.7 Mole Flow, lb-mol/hr. 100 Volume Flow, ACFM 236.808 Mass Flow, lb./hr. 1604.28 HEATER/COOLER TO LIQUID METHANE Temperature F. −259 Pressure Psia 14.7 Mole Flow, lb-mol/hr. 100 Volume Flow, ACFM 1.01307 Mass Flow, lb./hr. 1604.28

Methane gas dissociated at the sea floor flows through a 12-M pipe to the topside vessel where it is passed through a turbine to extract energy by simple expansion to atmospheric pressure. In this process the gas cools down to −258.676° F., which can be further liquefied by just cooling it to −259° F.

For calculating the amount of energy generated by the turbine, an example flow sheet was run assuming:

Flow rate: 100 ibmol/hr (0.9 MMSCFD)

Composition: 100% Methane

The gas at the inlet of the turbine is at 2033 psia and 58° F. When it is expanded at atmospheric pressure by passing through a turbine, 0.6 MMBTU/hr, of energy is generated. Current design suggests that around 500 MMSCFD of methane gas can be generated every day (24 hrs.), which results in 363 MMBTU/hr. (106.5 MW).

The environmental impact of gas hydrate harvesting is significantly reduced due to the boundary layer of decreasing pressure around the warm water spray. This boundary layer effectively repels any sea life with minimal impact as confirmed in laboratory experiments. Marine fife is moved out of the way when the edge of the boundary layer surrounding the warm water spray comes in contact with it. No physical harm occurs to the marine life since the slow moving boundary layer pressure increases as the warm water spray gets closer and safety lifts and conveys the marine life out of the way—similar to how a leaf blower moves leaves out of the way of the highest velocity stream without causing any physical damage to the leaves.

The pressure of the warm water sprays will be controlled according to each specific type of gas hydrate harvesting location to avoid any negative environmental impact on marine life. For solid formations of gas hydrates associated with marine seeps and vents, a lower pressure would be maintained and gentle, wide cone sprays used to avoid excessive flow rate of methane gas being harvested and avoid any negative impact on surrounding marine life. The warm water spray method will only remove solid gas hydrates on the upper most surface of the solid ice formations. The vast majority of these solid ice formations would remain behind along with the marine life known to flourish in these environments such as tubeworms. For applications of dispersed gas hydrates in marine sands with lower concentrations of marine life, the use of high-pressure warm water sprays may be safely utilized, as well. Ongoing monitoring of marine life would be done to insure that no marine life is being negatively impacted by the invention as claimed.

Storage of the methane gas on the topside vessel would be at near atmospheric pressure in liquid state as is the common design practice for storage of methane gas on seagoing vessels. The topside operations would be located immediately above the gas hydrate harvesting dome. Additional storage vessels could be docked nearby for receiving and transporting methane gas back to the mainland.

The size of the gas hydrate harvesting operation could be from small to large scale depending upon the desired investment and return desired. Multiple topside vessels could be used in adjacent paths to increase the overall production rates, which are variations of the same invention as claimed.

It is understood that modifications to the invention may be made as might occur to one skilled in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder that achieve the objects of the invention have not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims. 

1. A method for recovering methane from a hydrate associated with a marine sand in a selected area on the sea floor, comprising: placing a floating vessel at a selected location on the surface of the sea; using the vessel, placing a dome structure in proximity to the sea floor and over the hydrate; pumping sea water from the vessel through a water pipe to below the dome structure to contact the marine sand; capturing gas released from the hydrate under the dome structure; and flowing the gas released from the hydrate to a selected destination through a gas pipe.
 2. The method of claim 1 wherein the heated water is pumped to below the dome structure to contact the marine sand through a plurality of spray jet nozzles.
 3. The method of claim 1 wherein the flow rate of the gas through the gas pipe is controlled by a valve in the gas pipe.
 4. The method of claim 1 wherein the selected destination is the floating vessel.
 5. The method of claim 1 wherein the gas released from the hydrate is further flowed through a turbo-expander.
 6. The method of claim 5 wherein the gas flowed through the turbo-expander is further cooled to form liquefied gas.
 7. The method of claim 6 wherein the liquefied gas is stored on the vessel.
 8. The method of claim 5 wherein the turbo-expander is mechanically coupled to an electrical generator to generate electrical power.
 9. The method of claim 1 wherein the dome structure is propelled along the sea floor by directing a portion of the sea water pumped through the water pipe through a nozzle outside the dome structure.
 10. The method of claim 2 wherein the sea water is pumped through the water pipe and the plurality of spray jet nozzles at a rate greater than 10,000 gal/min.
 11. The method of claim 1 wherein the sea water is heated by a heater on the vessel to a temperature to allow the temperature of the water contacting the marine sand to be at a temperature of about 300 deg F. or more.
 12. A method for recovering gas from a gas seep at the sea floor, comprising: placing a floating vessel at a selected location on the surface of the sea; using the vessel, placing a dome structure in proximity to the sea floor and over the gas seep; capturing gas released from the gas seep under the dome structure; flowing the gas released from the gas seep to a selected destination through a gas pipe; and flowing the gas from the gas pipe through a turbo-expander.
 13. Apparatus for recovering methane from a hydrate associated with a marine sand in a selected area on the sea floor, comprising: a rigid dome structure adapted to be supported by a floating vessel and having a connection for a gas pipe and a valve to control flow through the gas pipe; a spray header for water within the dome structure; and spray jet nozzles connected with the spray header.
 14. The dome structure of claim 13 wherein the rigid dome structure comprises an array of domes connected by a connecting material.
 15. The dome structure of claim 13 further comprising an outer skirt.
 16. The dome structure of claim 13 wherein the rigid dome structure comprises stainless steel plate.
 17. The dome structure of claim 13 further comprising apparatus for propel the dome structure along the sea floor.
 18. Apparatus for liquefying gas from a hydrate associated with a marine sand, comprising: a floating vessel; a dome structure adapted for placing in proximity to the sea floor and over the hydrate; a pump on the vessel for pumping sea water from the vessel through a water pipe to below the dome structure to contact the marine sand; a gas pipe from the dome structure to the vessel; a valve for controlling flow through the gas pipe; and a turbo-expander connected to the gas pipe.
 19. The apparatus of claim 18 further comprising a heater on the floating vessel for heating the sea water.
 20. The apparatus of claim 18 further comprising apparatus on the vessel for cooling gas from the turbo-expander to liquefy the gas. 